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Unit 8: When Chemicals Meet Water—The Properties of Solutions

Section 4: Solution Concentrations

Frozen Concentrate

Figure 8-6. Frozen Concentrate

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Frozen Concentrate

Figure 8-6. Frozen Concentrate

A concentrate, like frozen concentrated orange juice, is a solution where most of the solvent (water) has been removed leaving primarily the solute (very potent orange juice). The concentrated orange juice is ready to be reconstituted by adding water.

In making and using solutions, a key question is how strong each solution should be—that is, how much solute should be mixed with the solvent. A parent needs to know just how much orange juice concentrate to add to water to make a refreshing drink. (Figure 8-6) To keep a pool clean, we need to know exactly how much chlorine to dissolve in a given amount of water. A doctor prescribing a drug for a patient needs to know exactly how much of that drug is contained in each pill or injection of medication. The amount of solute dissolved in a given amount of solution is called the concentration of the solution.

How the concentration is measured or described for a solution depends on the nature of the solutes in the solution, how concentrated the solution is, and what we plan on using this concentration for. The simplest concentrations we see are the ones we can find listed on the bottles of household chemicals that come in different strengths. These are usually given as percentages.

Mass and Volume Percentages

If we were to look in the bathroom or kitchen of our house, we would probably find a bottle of peroxide for disinfecting cuts or bleach for cleaning that have a percentage listed prominently on the front of the bottle. This percentage is a form of concentration to tell how strong that solution of peroxide or bleach is. Most household bleach will be listed at 5%. This is a mass percentage or (m/m) percent, or the (mass of the solute/mass of solution) x 100%. This means if we were to measure out 100 grams of bleach, 5% or 5 grams of that would be the bleach molecules (which is also called "sodium hypochlorite," which is a solid solute).

When two liquids are mixed together, the calculation is a bit different. This is called a "volume/volume percent solution" or (v/v) percent solution, which means that it is the (volume of the solute/volume of the solution) x 100%. Note that this is the percent volume of solute per volume of solution, not just per volume of solvent. The peroxide we find in our bathroom in a brown bottle is usually a 3% (v/v) solution, which means if we poured out 100 milliliters of it, it would contain about 3 milliliters of liquid hydrogen peroxide, with the remainder being the water solvent.

Molarity

For chemists in the lab, the most common way to measure concentration is in moles of solute per liter of solution, or molarity (designated by the letter M). One mole of solute in one liter of solution is said to be "1.0 M" or 1 molar. Molarity is commonly used by both chemists and biologists, and special containers called "volumetric flasks" exist in a variety of sizes to help scientists prepare solutions of a specific molarity that they need. (Figure 8-7)

Using a Volumetric Flask

Figure 8-7. Using a Volumetric Flask

© Wikimedia Commons, Creative Commons License 2.5. Author: Nuno Nogueira, 25 September 2007.

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Using a Volumetric Flask

Figure 8-7. Using a Volumetric Flask

Chemists want solutions of specific molarities, and they use volumetric flasks to accomplish this. The line etched into the stem of the flask represents a very precise volume if the liquid is exactly at that level. So, chemists will measure out how much solute they need and put it into the bottom of the flask, and then slowly add solvent until the total volume of the solution is exactly at that line. Now the molarity is known because the number of moles of solute are in exactly the volume of the solution written on the outside of the volumetric flask.

One of the problems with molarity, because it is based on the volume of the solution, is that it is temperature dependent. This is because all substances, including liquid solutions, expand and contract as they are heated and cooled. Because of this, there are two other common measures of solution concentrations that are independent of temperature. One of them is called "molality." This concentration is calculated by dividing the moles of the solute by the mass of the solvent in kilograms, since mass doesn't change when the temperature changes. Another unit used is called the "mole fraction." In the mole fraction, the moles of the solute are divided by the total moles of everything present, both solvent and solute. Mole fractions are interesting because they can only have values between 0 and 1. Molality and mole fractions are important for measuring some of the properties of solutions that will be discussed in the later sections of this unit.

Glossary

Concentration

The amount of solute in a given amount of solution.

Mass percentage or (m/m) percent

The mass of the solute divided by the mass of solution multiplied by 100%.

Molality

The concentration of dissolved solute in a solution, expressed as the number of moles of solute per kilogram of mass of the solvent.

Molarity

The concentration of dissolved solute in a solution, expressed as the number of moles of solute per liter of the solution.

Mole fraction

The fraction of a mixture that is comprised of a particular molecule. It is expressed as the number of moles of the molecule of interest divided by the total number of moles of all the molecules in the mixture.

Volume/volume or (v/v) percent solution

The volume of the solute divided by the volume of the solution multiplied by 100%.

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